Efficient and consistent wireless downlink channel configuration

ABSTRACT

A configuration for downlink signals in a wireless communication system, methods of configuring the downlink signals, apparatus for generating the downlink signals, and apparatus for receiving and processing the downlink signals are described herein. Downlink signals in a wireless communication system are reconfigured in series of frames, with each frame carrying a preamble that provides fast cell search and system acquisition. In particular, the preamble includes a primary preamble and a secondary preamble, where the primary preamble is common to all sectors in a base station and all base stations in a system and the secondary preamble is effectively unique to each base station, and may be further distinguished based on a sector basis. In addition, pilot signals are aligned with base stations to occur at the same time within a frame and the PN sequence values of the pilot signals are based on a cell identification an antenna identification, thereby enabling prediction of pilots transmitted by interferers or neighboring base stations from acquisition of secondary preambles. Also, the pilot bits are selectively assigned from a center of an operating band outward. Due to the pilot placement and pilot modulation, the scheme enables interference mitigation and channel estimation without knowing the frequency bandwidth, which is especially advantageous in broadcast channel systems.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 61/035,355 filed on Mar. 10, 2008, which is incorporated herein byreference in its entirety for all purposes.

This application is related to U.S. patent application Ser. No.12/265,435, filed on Nov. 5, 2008, entitled “Advanced Technology FrameStructure with Backward Compatibility,” which is hereby incorporated byreference in its entirety for all purposes.

BACKGROUND

I. Field of the Invention

The invention relates to the field of wireless communications. Moreparticularly, the invention relates to an efficient downlink framestructure for a wireless communication system.

II. Related Art

Wireless communication systems are often confronted with similarperformance issues. For example, a wireless communication systems needsto support many clients by enabling each of the clients to rapidlyacquire and process the information provided by the system.Additionally, the clients or subscriber stations need to be able toaddress parameters and factors that degrade performance, such asinter-cell interferences.

It is desirable for a wireless system to address various aspects such asthe above-mentioned performance issues in a manner that optimizes systemperformance.

BRIEF SUMMARY

A configuration for downlink signals in a wireless communication system,methods of configuring the downlink signals, apparatus for generatingthe downlink signals, and apparatus for receiving and processing thedownlink signals are described herein. Downlink signals in a wirelesscommunication system are reconfigured in series of frames, with eachframe carrying a preamble that provides fast cell search and systemacquisition. In particular, the preamble includes a primary preamble anda secondary preamble, where the primary preamble is common to allsectors in a base station and all base stations in a system and thesecondary preamble is effectively unique to each base station, and maybe further distinguished based on a sector basis. In addition, pilotsignals are aligned with base stations to occur at the same time withina frame and the PN sequence values of the pilot signals are based on acell identification an antenna identification, thereby enablingprediction of pilots transmitted by interferers or neighboring basestations from acquisition of secondary preambles. Also, the pilot bitsare selectively assigned from a center of an operating band outward. Dueto the pilot placement and pilot modulation, the scheme enablesinterference mitigation and channel estimation without knowing thefrequency bandwidth, which is especially advantageous in broadcastchannel systems.

One aspect includes a method of downlink signaling in a wirelesscommunication system. The method includes transmitting a preamble havingat least a portion that is common across multiple sectors and basestations within the wireless communication system, and transmitting apilot sequence that is aligned in time and frequency with at least onedistinct pilot sequence transmitted by a distinct base station.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, objects, and advantages of embodiments of the disclosurewill become more apparent from the detailed description set forth belowwhen taken in conjunction with the drawings, in which like elements bearlike reference numerals.

FIG. 1 is a simplified block diagram of an embodiment of a wirelesscommunication system.

FIGS. 2 a-2 b are simplified timing diagrams of an embodiment of adownlink frame structure including a preamble.

FIG. 3 is a time/frequency diagram illustrating an embodiment of adownlink frame in an OFDMA system.

FIG. 4 is a simplified time/frequency diagram illustrating an embodimentof a slot.

FIG. 5 is a simplified time/frequency diagram illustrating an embodimentof pilot placement within a slot.

FIG. 6 is a simplified functional diagram illustrating a pilot sequencePN generator.

FIG. 7 is a simplified functional block diagram of an embodiment of abase station.

FIG. 8 is a simplified functional block diagram of an embodiment of asubscriber station.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

A configuration of downlink signals for a wireless communication system,and in particular, downlink signals in an Orthogonal Frequency DivisionMultiple Access (OFDMA) wireless communication system are describedherein. The downlink signals and configurations described herein enableefficient system design, support for Multiple Input Multiple Output(MIMO) configurations, support for Intra-cell Interference Mitigation(IIM) techniques and support for new frame structures. Among otherfeatures, the downlink signal configuration can be configured to include(1) a primary universal preamble, (2) a secondary preamble that uses apredictable PN code based on the sector ID, (3) a fixed frequencybandwidth for both the primary and secondary preamble that isindependent of the bandwidth of the system (4) pilot signals which use apredictable PN code based on the sector ID and antenna ID, (5) pilotbits that remain consistently placed within a slot independent of thepermutation scheme used, or (6) some combination thereof.

The primary preamble can be a universal preamble. That is, the sameprimary preamble may be configured across all sectors in a base stationand all of the base stations within a wireless communication system. Useof a primary preamble which is universal to all sectors enables a clientstation to use macro-diversity to acquire the primary preamble.

The secondary preamble can be specific to each base station in an area,such that a client station will not typically have the ability toconcurrently observe two base stations using the same secondarypreamble. Thus, the secondary preamble can be considered as unique toeach base station from the perspective of a client station. Thesecondary preamble can also identify or otherwise be configured based ona sector of a base station, such that different sectors of the same basestation will transmit distinct secondary preambles.

In one embodiment, the secondary preamble can have over 600 uniquepossibilities of configurations. Of course, in other embodiments, thenumber of unique possible secondary preambles may be varied depending onthe specific system requirements. Thus, a base station or sectors of abase station would not need to share a secondary preamble unless thereare a greater number of base stations or sectors than the number ofpossibilities. In such a situation, the sector ID or base station ID canbe assigned such that the secondary preambles transmitted within aparticular coverage area is unambiguous. Because the secondary preambleis independent of the primary preamble, parallel processing may beimplemented during acquisition.

The bandwidth used to transmit the preamble, including the primarypreamble and the secondary preamble, can be fixed. Additionally, thebandwidth used to transmit the preamble can be the same or less than thebandwidth of a frame that carries the preamble. Where the frame has awider bandwidth than the preamble, the frequency band of the frame thatis not encompassed by the preamble can be used to support some otherpurpose, such as data transmission.

The pilot signals broadcast by each base station, and in each sector ofeach base station, can be predicted based on the identity of the basestation and the sector identity. For example, the pilot sequence for aparticular sector within a base station may be generated based on apseudo random sequence generator in conjunction with a cell ID thatidentifies both the base station and the sector. In some embodiments,the pilot sequence may also depend on an antenna configuration, such asthe number of antennas used to transmit the sequence. In one embodiment,the number of antennas can correspond to the number of diversityantennas, which may be different than a number of physical antennas.

The pilot signals transmitted from all base stations can be time andfrequency aligned or otherwise synchronized to occur at thesubstantially same time and frequency within a frame. A client stationwithin a coverage area supported by a first sector and first basestation can determine identification of one or more neighboring basestations from the shared messaging. Thus, because the client stationknows the identity and/or number of neighboring base stations, it canpredict the location and value of the pilots transmitted by eachneighboring base station. The ability to distinctly receive and detectsubstantially time aligned pilot tones greatly facilitates channelestimation. The ability to efficiently perform channel estimation canfacilitate implementation of Inter-cell Interference Mitigation (IIM)techniques.

In one embodiment, the bits of the pilot PN sequence can be assigned tothe various pilot subcarriers in a manner that facilitates reception andprocessing. The pilot PN sequence bits are assigned starting from themiddle of the frequency band working outward. The pilot sequence isreinitiated or otherwise reset at the beginning of each frame.

The pilot sequences can be predictable, for example, based on the sectorID and the antenna ID, which can be determined from the acquiredsecondary preamble. In some cases, because the client station may notknow the number of antennas used in the sector before decoding theinformation on a broadcast channel, pilot information in the broadcastchannel can be sent over a predetermined number of antennae, by use of apilot sequence generated based upon the predetermined number ofantennae, such as when there is a single antenna, it can be identifiedas antenna 0. Alternatively, multiple antennas could use the antenna 0PN sequence to determine the PN sequence for the broadcast channel.Because of this predictability, interference mitigation can be used onthe broadcast channel.

FIG. 1 is a simplified functional block diagram of a system 100implementing the downlink signal configuration described herein anadvanced technology frame structure. The wireless communication system100 includes a plurality of base stations, such as base stations 110-1and 110-2, coupled to a network 114, such as a wide area network. Eachbase station, e.g. 110-1, serves devices within its respective coveragearea, e.g., 112-1, sometimes referred to as a cell.

A first base station 110-1 serves a first coverage area 112-1 and asecond base station 110-2 serves a corresponding second coverage area112-2. The base stations 110-1 and 110-2 are depicted as adjacent orotherwise neighboring base stations for the purposes of discussion.

The coverage areas 112-1 and 112-2 may be sectorized. For example, thefirst base station 110-1 can be configured to use a plurality ofantennas to support a sectorized coverage area 112-1 having threedistinct sectors, 112-1 a, 112-1 b, and 112-1 c.

As an example, the base stations 110-1 and 110-2 serve those deviceswithin the respective coverage areas 112-1 and 112-2. As shown in FIG.1, first and second subscriber stations or client stations 120 a and 120b are within the first coverage area 112-1 and can be supported by thefirst base station 110-1.

Each of the first and second base stations, 110-1 and 110-2, can supportan efficient downlink signal structure, as described herein. Each of thebase stations, 110-1 and 110-2, can transmit a preamble that includes aprimary preamble and a secondary preamble. The primary preambles can beshared across the base stations 110 within the system, while thesecondary preambles may differ based on the base station ID and/orsector ID.

Each base station, e.g., 110-1 and 110-2 can also transmit a pilotsequence that is determined based on a pseudo random sequence generatorin conjunction with a base station ID value, a sector ID value, and anumber of antennas.

First and second subscriber stations or client stations 120 a and 120 bcan be configured to receive the efficient downlink signals and acquirethe frame timing, base station and sector ID values, and otherwisecommunicate based on the information in the efficient downlink signals.

The wireless system 100 can be an Orthogonal Frequency Division MultipleAccess (OFDMA) system, such as described in IEEE 802.16e. Additionally,the system 100 may utilize the frame structure of IEEE 802.16e or someother frame structure, such as an advanced frame structure proposed forIEEE 802.16m and described in related application U.S. patentapplication Ser. No. 12/265,435, filed on Nov. 5, 2008, entitled“Advanced Technology Frame Structure with Backward Compatibility.”

The wireless system 100 may alternatively support some other type ofcommunication systems, such as, but not limited to, a Long TermEvolution (LTE) system, or some variant of one or more of OFDMA,IEEE802.16, or LTE.

In the Application “Advanced Technology Frame Structure with BackwardsCompatibility,” new frame structures are described which incorporate an802.16m system within an exiting 802.16e system. The technology aspectsdescribed below may be applied to an 802.16m system, including one whichis deployed as a Greenfield system or one which is incorporated withinan existing 802.16e system. The technology aspects can also be appliedto other types of systems, including, but not limited to, a Long TermEvolution (LTE) system. LTE is the name given to a project within theThird Generation Partnership Project (3GPP) to improve the UniversalMobile Telephone Service (UMTS) mobile phone standard to cope withfuture requirements. Goals include improving efficiency, lowering costs,improving services, making use of new spectrum opportunities and betterintegration with other open standards.

FIGS. 2 a-2 b are simplified timing diagrams of an embodiment of adownlink frame structure including a preamble.

FIG. 2 a illustrates an advanced technology enabled base stationperspective of an advanced technology frame embodiment. In operation,the frame embodiment of FIG. 2 a can be supported by the systemillustrated in FIG. 1. The advanced technology enabled base station isable to support communications with subscriber stations or clientstations that are configured to receive either the legacy frames or theadvanced technology frames or both. Since both the legacy and theadvanced technology portions in the frames are multiplexed to the airlink in a time division manner, the advanced technology enable basestations can allocate or process data in resource allocations in each ofthe sub-subframes to support communications with all subscriberstations.

Each frame 201 can include a preamble 211 that includes a primarypreamble and secondary preamble. As shown in the timing diagram of FIG.2 a, the bandwidth of the preamble 211 may be narrower than thebandwidth of the frame 201. The bandwidth of the frame 201 may be fixedor may vary, but the bandwidth of the preamble 211 is fixed for allframes 201.

FIG. 2 b illustrates the advanced technology frame 203 at a time inwhich support for legacy communications has substantially beeneliminated. Nearly the entire downlink and uplink subframes arededicated to supporting advanced technology communications. Thus, theadvanced technology frame structure supports an orderly transition toadvanced technology, while maintaining support for legacy devices.

Regardless of the support, or lack thereof, for the legacy devices, eachframe 203 includes a preamble 211 that includes the primary preamble anda secondary preamble. As shown in the frames 203 of FIG. 2 b, thepreamble 211 is narrower in bandwidth than the frame 203 bandwidth. Inother embodiments, the bandwidths of the preamble 211 and frame 203 canbe substantially the same.

In an Orthogonal Frequency Division Multiple Access (OFDMA) system, thetransmitting station can use a Fast Fourier Transform (FFT) to create aregular array of subcarriers distributed across a frequency band. Eachsubcarrier is modulated to carry information, such as user data, controlinformation, pilot signaling and the like. The receiving station canalso use an FFT, IFFT, or some other transform to extract informationfrom the subcarriers.

For the purposes of example, some sample numerical values are includedherein. These values are intended to facilitate the description only.Many different numerical values could be used to implement variousaspects of this technology.

In the example herein described, a sample time for signaling in aIEEE802.16m system is defined as: T_(samp)=1/(12500×2048) s=0.0390625μs, yielding F_(s)=25.6 MHz. This corresponds to the sample time for a2048-point FFT with a subcarrier spacing Δf=12.5 kHz tone spacing. Theuseful symbol time is T_(b)=80 μs. This example supports a cyclic prefix(CP) of ⅛ or ¼ of a useful symbol time. A CP of ⅛ yields a guard time ofT_(g)=10 μs, yielding a symbol time of T_(s)=90 μs, and a CP of ¼ yieldsa guard time of T_(g)=20 μs, yielding a symbol time of T_(s)=100 μs.

FIG. 3 is a illustrative drawing showing a frame level representation ofa portion of an OFDMA frame 300 configured as a Green field deployment,as shown in FIG. 2 a. The principles can be directly applied to a systemwhich is deployed in conjunction with another system as shown in FIG. 2b. In FIG. 3, time 302 is shown on the horizontal axis and is increasingfrom left to right. Frequency 304 is shown on the vertical axis and isincreasing from bottom to top. In OFDMA, multiple access is achieved byassigning to an individual client station a group of OFDM tones. Thesetones are referred to herein as subcarriers. For example, a group ofsubcarriers 306 is indicated on FIG. 3 as an array of dots. In FIG. 3,the group of subcarriers 306 is five symbols wide as indicated by thefour symbol boundaries 310. The group of subcarriers 306 is 16subcarriers tall, as indicated by the 15 subcarrier boundaries 312.

A base station does not assign individual subcarriers for use by aspecific client station. Instead, the base station assigns thesubcarriers to a client station in groups of a fixed size. These groupsare referred to herein as slots and are often more generically referredto as allocation units. In FIG. 3, a slot 308 includes an array ofsubcarriers which is shown as a block that is five symbols wide and 16subcarriers tall.

A subchannel can be defined as a group of contiguous subcarrier rows,which according to the numeric example given above, includes 16subcarrier rows and spans 200 kHz in the frequency domain. A subchannelis measured in terms of frequency and extends beyond slot and frameboundaries. For example, Subchannel 316 is indicated in FIG. 3 with hashmarks and is designated as subchannel 1.

In addition, N_(sub) ^(DL) is used to represent the number ofsubchannels spanned by the downlink, and N_(sub) ^(UL) is used torepresent the number of subchannels spanned by the uplink. In thisexample, a number of subchannels can vary in integer steps from 6 to100. Typically the number of subchannels is determined based on thefrequency width of the available spectrum. The number of subchannels maybe different between the downlink and the uplink to allow for asymmetricFDD operation. In an asymmetric FDD system, the span of the spectrumallotted to the uplink and the downlink are different from one another.Such a system might be designed based on the availability of spectrum orbased on the premise that the expected data load for the uplink and thedownlink are different. Table 1 shows some exemplary values for a set ofpossible system band widths, assuming that N_(sub) ^(DL)=N_(sub) ^(UL).

TABLE 1 A comparison of occupied bandwidths. 802.16 m System Occupied.16e PUSC .16e FUSC .16e AMC BW BW Occupied Occupied Occupied (MHz)N_(sub) ^(XX) (MHz) BW (MHz) BW (MHz) BW (MHz) 1.25 6 1.20 0.93 1.171.19 3.50 16 3.20 3.29 3.34 3.38 5.00 23 4.60 4.60 4.67 4.74 7.00 336.60 6.57 6.65 6.76 8.75 42 8.40 8.21 8.31 8.45 10.00 47 9.40 9.20 9.319.46 20.00 94 18.80 18.39 18.63 18.91

The time duration of a slot is referred a slot time (h.). In FIG. 3, theslot time is 5 symbols wide. Slot time is a measure of time and extendsbeyond subchannel boundaries. For example, Slot time 320 is indicated inFIG. 3 with hash marks and is designated as slot time 6.

Using the definitions developed and described above, a slot can bedefined as an area in time and frequency that spans one subchannel byone slot time.

Assuming a frame is 5 ms in duration, using the exemplary numerologygiven above, Table 2 shows the number of slot times per frame for eachof the supported CP, as well as the amount of unused time at the end ofa frame.

TABLE 2 Slot timing. CP Slots/frame (M) Unused Time ⅛ 11 1280T_(samp) =50 μs ¼ 10 0

For FDD systems, the downlink slot times in one frame are assigned toone of several purposes including the downlink, a multicast andbroadcast service (MBS), and null (unused) slots. MBS is used to senduser data to more than one user at a time and could be used to providemobile television services. The uplink slot times are assigned to theuplink and null (unused) slots. For TDD systems, the slot times areassigned to the uplink, the downlink, MBS, and null slots.

Assume that frame 300 shown in FIG. 3 is configured as a TDD frame with8 subchannels and 10 slots, as shown. Thus, N_(sub) ^(DL)=N_(sub)^(UL)=N_(sub) ^(XX)=8. The subchannels are labeled from └−N_(sub)^(XX)/2┘ to └N_(sub) ^(XX)/2┘=[−4] to [4] with the lowest numberedsubchannel corresponding to the lowest frequency.

Assuming 16 subcarriers per subchannel, as a group, the 8 subchannelsspan the subcarrier rows from −8N_(sub) ^(XX) to 8N_(sub) ^(XX)=[−64] to[64]. Using n to be the subchannel of interest, the null subchannel 0(n=0) contains only the 0 (DC) subcarrier 324 and is not used. If n<0,then subcarrier n contains the subcarrier rows from 16n to 16(n+1)−1. Ifn>0, then subcarrier n contains the subcarrier rows from 16(n−1)+1 to16n.

FIG. 3 shows the numbering scheme for slots. Slots are identified by anordered pair (k,l), where k indicates the subchannel spanned by the slotand l indicates the slot time within a frame.

FIG. 4 is an expanded view of slot 308, which is designated slot (3,4).Individual subcarriers are identified by a ordered quadruple (k, l, m,n), where (k,l) indicates the slot within a frame, m=0, . . . , 15indicates the subcarrier within the slot (k,l), and n indicates thesymbol within the slot (k,l). Thus, subcarrier 402 is designated assubcarrier (3,4,8,2). The subcarrier rows are also indicated on FIG. 4.Slot 308 spans from subcarrier row 33 to subcarrier row 48. Subcarrier402 is in subcarrier row 41.

The first slot time in the frame is typically allocated to the downlinkand contains synchronization and informational signals used to acquirethe system. The other slot times can be divided between the uplink, thedownlink channel, MBS, and null slots as needed.

The downlink consists of several related portions: the downlinkpreambles, the downlink pilots, and the downlink channels. The downlinkpreamble is sent at a fixed, known interval, and is used by the clientstations to perform system acquisition. The downlink pilots are used bythe client stations to do channel estimation. The downlink pilots areinterspersed among the data carriers or otherwise sparsely seeded amongthe subcarriers of a slot.

The pilot pattern can vary based on the channel or portion of a frame inwhich the pilot is transmitted, the number of antennas available to thesystem, or some combination thereof.

The downlink channel portion typically includes several channel types,including the physical broadcast channel, physical downlink datachannel, physical downlink control channel, physical multicast channeland the like.

According to one aspect of the invention, the preamble is partitionedinto two pieces, namely the primary preamble and the secondary preamble.The primary preamble is used by the client station to identify frametiming. In one embodiment, the number of subchannels that the primarypreamble spans remains fixed independent of the number of subchannelswhich are used by the system. In an IEEE 802.16e compliant system, thepreamble spans an entire symbol, thus spanning across every subchannelavailable for use in the system. Upon initial entry into a system, theclient station does not know how many subchannels are being used by thesystem. But even without this information, the client station knows thesize of the preamble which helps speed system acquisition.

As noted with respect to FIG. 1, a base station may be comprised ofmultiple sectors. According to one aspect, all sectors of every basestation in the system transmit the same primary preamble. Assuming thesectors are time synchronized, the use of a common primary preambleprovides macro-diversity, enabling easy acquisition of the frame timingas well as location of the center of the transmit band. According to802.16e, there is no common preamble used by multiple base stationsectors and, thus, the client stations cannot use macro-diversity todetect the preamble.

Continuing with the numerical example given above, the primary preambleconsistently occupies subchannels −3 to 3 in symbol 2 of slot time 0within a frame, no matter how many subchannels make up the frame. Of the96 subcarriers which make up subchannels −3 to 3, only the subcarrierscorresponding to subcarrier rows −41 to 41 are occupied. The othersubcarriers within these subchannels have no energy placed upon them.The DC subcarrier may also be vacant or otherwise have no energy placedon it.

Although other modulation schemes can be used, by way of example, theoccupied subcarriers in this example are modulated with a frequencydomain Zadoff-Chu sequence based on the nth roots of unityp_(u)(x)=exp(−jπux(x+1)/83) for x=0, 1, . . . , 83. The mapping of p(x)onto subcarriers is given by:

${f\left( {k,m} \right)} = \left\{ {{\begin{matrix}{{{16\mspace{14mu} k} + m}\mspace{14mu}} & {{{if}\mspace{14mu} k} < 0} \\{0\mspace{14mu}} & {{{if}\mspace{14mu} k} = 0} \\{{{16\mspace{14mu} k} + m - 15}\mspace{11mu}} & {\;{{{if}\mspace{14mu} k} > 0}}\end{matrix}{{subcarrier}\left( {k, 0, m, 2} \right)}} = \left\{ \begin{matrix}{p_{1}\left( \left\lbrack {{f\left( {k,m} \right)} + 41} \right\rbrack \right)} & {\mspace{14mu}{{{if}\mspace{11mu} - 41} \leq {f\left( {k,m} \right)} < {0\mspace{14mu}{{or}0}} < {f\left( {k,m} \right)} \leq 41}\mspace{14mu}} \\0 & {Otherwise}\end{matrix} \right.} \right.$

In one aspect, the secondary preamble is mapped to the identity of aparticular sector. Thus, a client station can identify a base stationsector by detecting the secondary preamble.

In one aspect, the number of subchannels that the secondary preamblespans remains fixed independent of the number of subchannels that areused by the system. According to 802.16e, the preamble spans an entiresymbol, thus spanning across every subchannel available for use to thesystem. The client station does not know how many subchannels are beingused in the system when it first determines the sector identity based onthe secondary preamble. Knowing the span of the secondary preamble aidsthe determination of the sector identity. According to another aspect,the secondary preamble spans the same number of subchannels as theprimary preamble. However, in yet other aspects, the secondary preamblemay span a greater number of subchannels such as, for example, to aid inchannel estimation.

Continuing with the numerical example given above, each sector isassigned an identity number from 0 to 624. The sectors transmit afrequency domain Zadoff Chu sequence based on the identity number.According to one aspect, the secondary preamble always occupiessubchannels −3 to 3 in symbol 3 of slot time 0 within a frame. Of these96 subcarriers, only subcarriers −41 to 41 are occupied, the othersubcarriers within these subchannels having no energy placed upon them.Of course, the DC subcarrier can also be vacant. With N_(cell) ^(ID)representing the sector identity, the subcarrier modulation is providedby:

${{Subcarrier}\left( {k, 0, m, 3} \right)} = \left\{ {\begin{matrix}{p_{{\lfloor{N_{CELL}^{ID}/25}\rfloor} + 16}\left( {\left\lbrack {{f\left( {k,m} \right)} + 41 + {3\left( {N_{CELL}^{ID}{mod}\; 25} \right)}} \right\rbrack{mod}\; 83} \right)} & {{{if} - 41} \leq {f\left( {k,m} \right)} < {0\mspace{31mu}{or}\mspace{14mu} 0} < {f\left( {k,m} \right)} \leq 41} \\0 & {Otherwise}\end{matrix}\begin{matrix}\; \\{\{\}}\end{matrix}} \right\}$

Since a time delay corresponds to a frequency shift in the Zadoff-Chusequence, the parameters in this formula have been selected so thatsectors can be uniquely identified in the case of expected time ofarrival differences between BSs. The cross-correlation between any twodifferent Zadoff-Chu sequences, even after puncturing for a DC carrier,is still approximately −9 dB below the correlation peak, even againstshifts in frequency.

FIG. 5 shows one slot of the downlink which indicates the subcarrierswhich carry the pilot bits as a function of the antenna port. In theexemplary embodiment, the system supports transmission on 1, 2, or 4diversity antennas which are associated with a single sector. If onlyone antenna is used, it is designated as antenna 0. If 2 antennas areused, they are designated as antennas 0 and 1. If 4 antennas are used,they are designated as antennas 0, 1, 2, and 3. Each antenna has aunique placement of its pilots, one example of which is shown in FIG. 5.

According to FIG. 5, antenna 0 places pilots at subcarriers 4 and 12 onsymbols 1 and 3 respectively of a slot. Antenna 1 places pilots atsubcarriers 4 and 12, but on symbols 0 and 2 of a slot. Similarly,antennas 3 and 4 use subcarriers 0 and 8, with antenna 3 placing them insymbols 1 and 3, and antenna 4 placing them in symbols 0 and 2. Notethat pilots are not placed in tones that are already occupied by thepreamble.

According to one aspect, the same pilot pattern is used on both thedownlink and the uplink. On the uplink, the client station often usesonly 1 subchannel and at most 2 antennas. Because the base station andthe client stations can more accurately use interpolation rather thanextrapolation, the pilot signal location for antennas 0 and 1 is in thecenter portion of the slot.

According to the aspect described above, the pilots are consistentlyassigned to the same subcarriers within a slot, regardless of which basestation sector is transmitting them and the permutation scheme beingemployed. In addition, the PN sequence used to modulate the pilotsignals is tied directly to the base station sector identity. Knowingthe location of the pilot bits within the slots and the sector identity,the client stations can more readily track the pilot signals receivedfrom interfering sectors. Doing so makes channel estimation lessburdensome and more accurate.

Continuing with the numerical example given above, the pilot codes arebinary phase shift keying (BPSK) modulated by a 32767-bit M-sequence,generated from the polynomial p(x)=x¹⁵+x⁷+x⁴+x+1. FIG. 6 shows anembodiment of a PN generator that creates such a sequence. The PNgenerator is initialized at the start of each frame with the sectoridentity N_(cell) ^(ID) and the antenna port p. For example, the 9 bitidentity number is inserted into registers x¹ to x¹⁰ and the two bitantenna port number is inserted into registers x¹¹ and x¹².

Using the subcarrier designations we developed above, the subcarriersare designated with an ordered quadruple (k, l, m, n), where k is thesubchannel, l is the slot time within a frame, m is the subcarrierwithin the slot (k,l), and n is the symbol within the slot (k,l). Thepilot bits produced by the PN generator are assigned to slots inincreasing absolute value of k and then l. Thus as the bits are producedby the sequence generator, they are assigned in groups of 4 to each slotin the first column, such as slots (1,0), (−1, 0), (2,0), (−2,0) etc.,until all subchannels of slot time 0 have been assigned.

Within a slot, 4 pilot bits are assigned first in order of increasing m,and then in order of increasing n. Thus, within these slots, for antenna0, the first four pilot bits are inserted into subcarriers (1, 0, 4, 1),(1, 0, 12, 1), (1, 0, 4, 4), (1, 0, 12, 4) respectively.

The process then moves on to assign pilot bits to slots within slot time1, such as (1,1), (−1,1), (2,1), (−2, 1), etc. This process continuesuntil all slots in a frame have been assigned pilot bits. The decisionto place a pilot bit in a particular position within a particular slotis made on a per tone (subcarrier) basis. For example, the pilot tonesmay be excluded from those subcarriers carrying preamble information.

Pilots are assigned in this way so that the client station can know whatthe pilot codes are for the broadcast and assignment channels,regardless of the number of subchannels.

Compared with other system embodiments utilizing distinct downlinksignal configurations, such as 802.16e systems, the embodimentsdescribed hereinabove provide several advantageous features. For exampleaccording to 802.16e, it was not possible to predict either the locationor the value of the pilot bits coming from a neighboring base stationsector based solely on the sector identity. According to 802.16e, thepilot sequence is not generated based solely on sector identity andantenna port number. In addition, the placement of the pilot bits withina slot can vary, such as, based on the permutation scheme which is beingused. According to certain aspects described herein, the location of thepilot bits within the slot remains consistent independent of thesevariables. Because both the pilot sequence and the pilot bit locationdescribed herein can be predicted based solely on the sector identityand the antenna port number, a client station can more easily determinethe interference that it is experiencing from neighboring sectors.

The preceding example of pilot placement is given only by way ofexample. Other sequences and sequence generation methods may be used.For example, the example given above would need to be modified if thesystem includes an odd number of subchannels (i.e. N_(sub) ^(XX) isodd.) In such a system, pilot bits could be assigned based on half slotsrather than full slots. In addition, the fixed placement of the pilotbits within a slot may vary from system to system. For example, a newscheme for the fixed placement of the pilot bits may be determined ifthe number of symbols per slot time changes.

In addition, in an alternative aspect of the invention, the primary andsecondary preambles may be placed in a slot which occurs later in theframe, which placement in some circumstances may have an advantageouseffect on system acquisition time.

The Physical Broadcast Channel (PBCH) can be a channel that is used toconvey global system parameters to the client stations. For example, itcarries system configuration information to be heard by all clientstations in the system. The information packets sent over the broadcastchannel are generally short, and are repeated infrequently.

Continuing with the numerical example given above, the physicalbroadcast Channel occupies subchannels −3 to 3 in symbols 0 and 1 ofslot time 0 within a frame. Each frame contains one physical layerbroadcast packet, which carries one or more complete MAC layer broadcastpackets. The location of the broadcast channel in time and frequency,when combined with the pilot and preamble structure, allows fordemodulation of the preamble with interference mitigation techniquesusing only the information obtained from the preamble search.

FIG. 7 is a simplified functional block diagram of an embodiment of abase station 110, which can be, for example, a base station in thesystem of FIG. 1.

The base station 110 includes a transmitter 760 and receiver 710 coupledto an antenna 702. The receiver 710 is configured to receive the uplinksignals from one or more subscriber stations, for example, based onuplink resource allocations provided by the base station 110 in thedownlink.

The downlink path includes a data source 730 that includes the bits orencoded symbols that are to be transmitted on the downlink resourceallocations to the various subscriber stations within the coverage areasupported by the base station 110 using the antenna 702.

The data source 730 is coupled to an input of an advanced resourcemapper 742 that can be configured, for example, to support a legacyframe structure or a frame structure such as illustrated in FIGS. 2 aand 2 b.

A pilot PN generator 720 is configured to generate the pilot PNsequence, based, for example, on a base station ID, sector ID, or someother identifying parameter, or some combination thereof. The output ofthe pilot generator 720 can be coupled to another input of the scheduler770.

A preamble generator 722 can be configured to generate the primarypreamble as well as the quasi-unique secondary preamble. The preamblegenerator 722 can couple the primary and secondary preambles to thescheduler 770. The preamble generator 722 can generate the universalprimary preamble, for example, based on a look up table or a generatormodule. The preamble generator 722 can generate the secondary preamblein a similar fashion. For example, the preamble generator 722 can storethe unique secondary preamble in a look up table in memory or cangenerate the secondary preamble using the Zadoff-Chu sequence, sectorID, some other identifying parameter, or some combination thereof.

The scheduler 770 can be coupled to or otherwise synchronized to asystem clock 772. The scheduler 770 can selectively couple the pilot PNor preambles to the resource mapper 742 based on the system timeprovided by the system clock 772. The scheduler 770 can also reset thepilot PN generator 720 based on the system clock 772, for example, atthe frame boundaries.

FIG. 8 is a simplified functional block diagram of an embodiment of asubscriber station 120, alternatively referred to above as a clientstation. The subscriber station 120 can be, for example, the subscriberstation 120 in the system of FIG. 1.

The subscriber station 120 includes a receiver configured to receive theefficient downlink signals and process the signals for acquiring,synchronizing, and managing communications in the wireless communicationsystem.

The subscriber station 120 includes an antenna 806 through which theuplink and downlink signals are communicated. The antenna 806 couplesthe downlink signals to a transmit/receive (T/R) switch 810. The T/Rswitch 810 operates to couple the downlink signals to the receiver ofthe subscriber station 120 during a downlink subframe and operates tocouple uplink signals from the transmitter portion of the subscriberstation 120 during an uplink subframe.

During the downlink portion or subframe, the T/R switch 810 couples thedownlink signals to a receive RF front end 820. The receive RF front end820 can be configured, for example, to amplify, frequency convert adesired signal to a baseband signal, and filter the signal. The basebandsignal is coupled to a receive input of a baseband processor 840.

The receive input of the baseband processor 840 couples the receivedbaseband signal to an Analog to Digital Converter (ADC) 852 thatconverts the analog signal to a digital representation. The output ofthe ADC 852 can be coupled to a receive filter 853 that can beconfigured to substantially limit out of band noise and interference.The output of the receive filter 853 is coupled to a transformationmodule, such as Fast Fourier Transform (FFT) engine 854 that operates toconvert the received time domain samples of an OFDM symbol to acorresponding frequency domain representation. The sample period andintegration time of the FFT engine 854 can be configured, for example,based upon the downlink frequency bandwidth, symbol rate, subcarrierspacing, as well as the number of subcarriers distributed across thedownlink band, or some other parameter or combination of parameters.

The output of the FFT engine 854 can be coupled to a downlinkchannelizer 856 that can be configured to extract the subcarriers fromthose symbols that are allocated to the particular subscriber station120. The downlink channelizer 856 can be configured, for example, toextract the portion of the legacy or enhanced downlink sub-subframes forwhich the subscriber station 120 is allocated. The output of thedownlink channelizer 856 can be coupled to a destination module 858. Thedestination module 858 represents an internal destination or output portto which received data may be routed.

The subscriber station 120 also includes a preamble decoder 870configured to access the sampled downlink signals and acquire theprimary preamble and unique secondary preamble. The subscriber stationcan also include a pilot decoder 880 configured to access the framesynchronized downlink samples to extract the pilot sequence. The pilotdecoder 880 can also generate one or more channel estimates based on thedecoded pilot signals. The preamble decoder 870 and the pilot decoder880 can be coupled to the channelizer 856 to control the extraction ofthe data in the allocated downlink resources. Similarly, the preambledecoder 870 and the pilot decoder 880 can be coupled to the uplinkchannelizer 864 to control the allocation of uplink data to theappropriate allocated uplink resources.

The uplink path is complementary to the downlink signal path. A sourcemodule 862 of the base band processor 840, which may represent aninternal data source or an input port, generates or otherwise couplesuplink data to the baseband processor 840. The source 862 couples theuplink data to an uplink channelizer 864 that operates to couple theuplink data to appropriate uplink resources that are allocated tosupport the uplink transmission.

The output of the uplink channelizer 864 is coupled to an IFFT engine866 that operates to transform the received frequency domain subcarriersto a corresponding time domain OFDM symbol. The uplink IFFT engine 866may support the same bandwidth and number of subcarriers as supported bythe downlink FFT engine 854.

The output of the uplink IFFT engine 866 is coupled to a transmit filter867 that shapes the bandwidth and removes out of band noise. The outputfrom the transmit filter 867 is coupled to a Digital to Analog Converter(DAC) 868 that converts the digital signal to an analog representation.The analog baseband signal is coupled to a transmit front end 822, wherethe signal is frequency translated to the desired frequency in theuplink band. The output of the transmit front end 822 is coupled to theT/R switch 810 that operates to couple the uplink signal to the antenna806 during the uplink subframe.

An LO 830 is coupled to a switch 832 or demultiplexer that selectivelycouples the LO 830 to one of the receive front end 820 or transmit frontend 822 so as to be synchronized to the state of the T/R switch 810.

Methods and apparatus are described herein for efficient downlinksignaling, generating of efficient downlink signals, and receivingefficient downlink signals in a wireless communication system.Additional related information can be found in related U.S. patentapplication Ser. No. 12/265,435, entitled “Advanced Technology FrameStructure with Backward Compatibility,” which is incorporated byreference herein in its entirety.

As used herein, the term coupled or connected is used to mean anindirect coupling as well as a direct coupling or connection. Where twoor more blocks, modules, devices, or apparatus are coupled, there may beone or more intervening blocks between the two coupled blocks.

The steps of a method, process, or algorithm described in connectionwith the embodiments disclosed herein may be embodied directly inhardware, in a software module executed by a processor, or in acombination of the two. The various steps or acts in a method or processmay be performed in the order shown, or may be performed in anotherorder. Additionally, one or more process or method steps may be omittedor one or more process or method steps may be added to the methods andprocesses. An additional step, block, or action may be added in thebeginning, end, or intervening existing elements of the methods andprocesses.

The above description of the disclosed embodiments is provided to enableany person of ordinary skill in the art to make or use the disclosedembodiments. Various modifications to these embodiments will be readilyapparent to those of ordinary skill in the art, and the genericprinciples defined herein may be applied to other embodiments withoutdeparting from the scope of the disclosure.

1. A method of downlink signaling in a wireless communication system,the method comprising: transmitting a primary preamble that is commonacross multiple sectors of a base station and multiple base stationswithin the wireless communication system, wherein the primary preambleis useable by the subscriber station for a system acquisition;transmitting a secondary preamble, wherein the secondary preamble isuseable by the subscriber to predict a pilot sequence; and transmittingthe pilot sequence that is aligned to occur at the same time andfrequency as at least one distinct pilot sequence transmitted by adistinct base station, wherein the pilot sequence is useable by thesubscriber station for channel estimation.
 2. The method of claim 1,further comprising: generating the secondary preamble based on at leastone identity selected from a group consisting essentially of a cell ID,a base station ID, a sector ID, and an antenna ID.
 3. The method ofclaim 2, further comprising: generating the pilot sequence based on theat least one identity.
 4. The method of claim 1, wherein the secondarypreamble is based on a Zadoff Chu sequence selected from a family ofZadoff Chu sequences.
 5. The method of claim 4, further comprising:mapping the Zadoff Chu sequence to at least one identity, wherein the atleast one identity is selected from a group consisting essentially of acell ID, a base station ID, a sector ID, and an antenna ID.
 6. themethod of claim 5, further comprising: generating the pilot sequencebased on the at least one identity.
 7. A method of downlink signaling ina wireless communication system, the method comprising: receiving theprimary preamble that is common across multiple sectors of a basestation and multiple base station within the wireless communicationsystem; synchronizing to the wireless communication system based on theprimary preamble; receiving a secondary preamble; predicting a pilotsequence based on the secondary preamble; receiving the pilot sequencethat is aligned to occur at the same time and frequency as at least onedistinct pilot sequence transmitted by a distinct base station; andestimating channel conditions based on the pilot sequence.
 8. The methodof claim 7, further comprising: determining at least one identityselected from a group consisting essentially of a cell ID, a basestation ID, a sector ID, and an antenna ID, based on the secondarypreamble.
 9. The method of claim 8, wherein the predicting the pilotsequence is based on the at least one identity.
 10. The method of claim7, further comprising: determining a Zadoff Chu sequence, from a familyof Zadoff Chu sequences, based on the secondary preamble.
 11. The methodof claim 10, further comprising: determining at least one identityselected from a group consisting essentially of a cell ID, a basestation ID, a sector ID, and an antenna ID, based on the Zadoff Chusequence.
 12. The method of claim 11, wherein the predicting the pilotsequence is based on the at least one identity.
 13. A base stationconfigured for use in a wireless communication system, the base stationcomprising: a transmitter configured to: transmit a primary preamblethat is common across multiple sectors of a base station and multiplebase stations within the wireless communication system, wherein theprimary preamble is useable by a subscriber station for systemacquisition; transmit a secondary preamble, wherein the secondarypreamble is useable by the subscriber station to predict a pilotsequence; and transmit the pilot sequence that is aligned to occur atthe same time and frequency as at least one distinct pilot sequencetransmitted by a distinct base station, wherein the pilot sequence isuseable by the subscriber station for channel estimation.
 14. The basestation of claim 13, wherein the transmitter is further configured to:generate the secondary preamble based on at least one identity selectedfrom a group consisting essentially of a cell ID, a base station ID, asector ID, and an antenna ID.
 15. The base station of claim 14, whereinthe transmitter is further configured to: generate the pilot sequencebased on the at least one identity.
 16. The base station of claim 13,wherein the transmitter is further configured to: generate the secondarypreamble based on a Zadoff Chu sequence selected from a family of ZadoffChu sequences.
 17. The base station of claim 16, wherein the transmitteris further configured to: map the Zadoff Chu sequence to at least oneidentity, wherein the at least one identity is selected from a groupconsisting essentially of a cell ID, a base station ID, a sector ID, andan antenna ID.
 18. The base station of claim 17, wherein the transmitteris further configured to: generate the pilot sequence based on the atleast one identity.
 19. A subscriber station configured for use in awireless communication system, the subscriber station comprising: areceiver configured to: receive a primary preamble that is common acrossmultiple sectors of a base station and multiple base stations within thewireless communication system; synchronize to the wireless communicationsystem based on the primary preamble; receive a secondary preamble;predict a pilot sequence based on the secondary preamble; receive thepilot sequence that is aligned to occur at the same time and frequencyas at least one distinct pilot sequence transmitted by a distinct basestation; and estimate channel conditions based on the pilot sequence.20. The subscriber station of claim 19, wherein the receiver is furtherconfigured to: determine at least one identity selected from a groupconsisting essentially of a cell ID, a base station ID, a sector ID, andan antenna ID, based on the secondary preamble.
 21. The subscriberstation of claim 20, wherein the receiver is further configured to:predict the pilot sequence based on the at least one identity.
 22. Thesubscriber station of claim 19, wherein receiver is further configuredto: determine a Zadoff Chu sequence, from a family of Zadoff Chusequences, based on the secondary preamble.
 23. The subscriber stationof claim 22, wherein the receiver is further configured to: determine atleast one identity selected from a group consisting essentially of acell ID, a base station ID, a sector ID, and an antenna ID, based onZadoff Chu sequence.
 24. The subscriber station of claim 23, wherein thereceiver is further configured to: predict the pilot sequence based onthe at least one identity.